This invention relates in general to athletic running and playing surfaces, specifically to a composite construction particularly adapted to running tracks and level playing fields for football and the like where it is desired to optimize the speed of a runner on the surface while at the same time reducing the likelihood of injuries.
Spring mounted floors or platforms have been known for a long time. U.S. Pat. Nos. 1,509,750; 1,747,352 and 2,167,696, for example, demonstrate small, special purpose devices that depress to some degree when a person stands on them. While some are related to athletics, none are designed for normal running. Known athletic surfaces such as running tracks, football fields, and playing surfaces for a wide variety of other sports typically have a conventional uniform construction including plain ground, grass over the ground, asphalt, gravel, and more recently, a layer of a resilient synthetic plastic material laid over the ground or concrete. These plastic materials, such as the products sold under the trade designation "Astroturf", "Chem-Turf", or "Tartan" are generally uniform in composition and thickness when applied to playing fields.
While these plastic surfaces enjoy a high degree of commercial success, they are not entirely satisfactory. First, they are comparatively expensive to install, particularly in the thick layers necessary to achieve large compliance. Second, in thin layers, such as 3/8 to 3/4 inch, the running surface has been found to produce a relatively high level of injuries. When the plastic layers are thick enough to significantly reduce injuries, they are poor running surfaces due to a relatively low horizontal shear modulus and a high dependence of vertical compliance on the foot contact area, e.g., full foot versus only toe or heel contact.
Surfaces specifically designed for running such as indoor and outdoor running tracks suffer from many of the difficulties enumerated above. Outdoor tracks are typically asphalt, clay, gravel deposited over the ground, or an artificial surface over concrete or some substantially rigid substratum. They are characteristically rigid and result in a usual high incidence of injuries such as shin splints and foot injuries due to the high collision forces generated by the human leg striking a rigid surface when running. These problems are, of course, accentuated for competitive runners and those who may not be in their best physical condition. Resilient layers on tracks, whether outdoors or indoors, may reduce injuries, but the principal uses of such surfaces is as a thin top layer to provide traction, reduce injury to the track from the track shoe spikes, and to provide a surface which is essentially maintenance-free.
Conventional construction for indoor running tracks uses an extremely stiff running surface laid on "sleepers" or elongated support members. Older tracks use stiff lengths of hardwood fixed to the stringers. Some more recent tracks have used other surface materials such as plywood overlaid with the resilient materials as discussed above. In either case, it has been assumed heretofore that the track surface should be generally rigid or highly stiff to yield the best possible running speeds on the track. Some other tracks have used plywood panels supported on several 2".times.4" beams arranged perpendicular to the running direction. Such tracks provide some degree of vertical compliance, but their mechanical response (i.e. vertical compliance) varies greatly depending on whether or not a runner lands over a support beam or "sleeper".
U.S. Pat. Nos. 1,693,655 and 3,114,940 describe floors formed by conventional, interfitted (i.e. tongue-in-groove) floor boards that are supported over rigid subfloors on sleepers that in turn rest on a layer of yielding material. In the '655 system, the yielding material is a pad or cushion of felt or an equivalent held in a U-shaped support bracket. While this arrangement does provide some "give" to the floor, it is not suitable for use as an athletic surface, particularly a running track. First, the yielding material is designed to absorb energy from a runner, not return it to him. Second, the compliance of the surface is not uniform. If the runner's foot lands over a sleeper it is more rigid than if the foot lands between the sleepers. Third, the floor boards are not independently sprung, and hence feed energy from one place on the surface to another. For example, the impact of one runner can develop an upward movement of the floorboard at an adjacent point which exchanges energy between the runners. Finally, there is no appreciation in the prior art of a general mechanical interrelationship between the optimal vertical compliance of a running surface and the running speed attainable on that surface other than the long accepted understanding that the hardest track surfaces produce the fastest speeds.
The '940 patent provides a rubber pad secured by staples to sleepers that support a rigid, hardwood upper floor. The pad rests directly on the floor and grooves in the pad resist a horizontal shift of the pad or the floor with respect to a concrete subfloor. This system provides a floor structure with a relatively large apparent mass. As a result, there is no enhancement of running speed or reduction of injuries associated with running. Also, as with the '655 construction, the floorboards are not independently sprung so that they feed energy across the floorboards.
It is therefore a principal object of this invention to provide a construction for an athletic playing surface that enhances the running speed of athletes performing on the surface.
Another object is to provide a playing surface that reduces injuries to athletes performing on the surface.
Still another object is to provide a playing surface with the foregoing advantages that has a response over its surface that is highly uniform and independent of the foot contact area.
A further object is to provide a playing surface with the foregoing advantages that has a low cost of construction and a high degree of longevity.
Yet another object is to provide a playing surface that is substantially free of vibrational cross talk between different areas of the surface to avoid interaction between athletes.
Still another object is to provide a surface that can be used in a wide variety of environments and for a wide variety of sports.